A new Earth?

Almost every week now, planet hunters are discovering new worlds, not in our solar system but in the far reaches of our galaxy. So how close are astronomers to finding a planet that supports life? Ian Sample reports

Andrew Collier Cameron stretches a hand towards the charcoal skies that have gathered over the Kingdom of Fife and talks of a planet that lies far beyond the clouds. It is a planet that has never been seen, even through the most powerful space telescopes.

It revealed itself to Cameron only by casting the tiniest of shadows as it strayed in front of its parent star, 1,000 light years from his office in the physics building at St Andrews University on the east coast of Scotland.

Though distant, the planet has given up a few of its secrets. It is a world that shows only one face to its sun: half of it basks in permanent warmth while the other lies in eternal darkness. It moves so swiftly round its star that a year there lasts only a few Earth days. It is a giant that would dwarf not just our blue-green planet but even Jupiter, the largest in the solar system, yet it is incredibly light in comparison: it has the density of willow. "It would float!" says Cameron, his fingers spread wide. "If you could only find a bath big enough!"

Here on Earth, Cameron and his team are part of a new wave of astronomers who are scouring the galaxy for glimpses of unknown worlds and, in recent years, their success has been astounding. While our own solar system lost a planet last year, thanks to the unceremonious demotion of Pluto from planet to "dwarf planet", Cameron and his peers have found more than 250 new worlds, in solar systems far, far away. With almost every week that goes by, new discoveries are made. This evening, in fact, planet hunters in America are to declare a major coup with the announcement from Nasa of a newly discovered solar system that has striking similarities to our own.

Individually, each new planet we discover adds a fragment to our understanding of the galaxy beyond our celestial backyard. But collectively, these scattered worlds point to something more profound. Scientists' ability to spot distant planets has accelerated more rapidly in the past 20 years than many dared imagine. And, for the first time, they are now talking seriously about searching for a planet that could recast the role of humanity in the universe: a second Earth.

"There's a very strong innate desire in humans to know how special our planet is," says Cameron. "There's a lot of talk about how finely tuned Earth is for life, but just how special is the Earth? How special is life? Are we alone? This is what gets people get into this."

So far, nearly all the planets discovered beyond the solar system are enormous gas giants, like our own Jupiter, only hotter and bigger. They orbit so blisteringly close to their suns that they carve out a year in a just a few Earth days, their surfaces approaching a searing 2,000C.

These huge worlds are the easiest to spot, though the vast majority are still detected indirectly, through the effects they have on their parent stars. The most successful technique to date relies on the famous Doppler effect, which affects all kinds of energy waves - from sound waves to waves of light. (A familiar example of the Doppler effect is that when a police car passes you by, its siren drops in pitch. That is because as the car is coming towards you, the soundwaves get compressed, which makes the noise higher-pitched, and as the car moves away, the waves stretch out, which lowers the pitch.)

When a planet swings around a star, its gravitational field has an impact on the star itself: it makes the star move in a little circle of its own. If the planet is big enough, astronomers can pick up this cosmic dance by peering at the starlight that reaches Earth tens, hundreds, even thousands of years later. If the starlight shifts to a more reddish colour, the light waves are being stretched out, so the star must be being pulled away from Earth. As the star is pulled back towards us, the light we see turns more bluish, as the waves are squashed together. By watching the shift from red to blue and back again, astronomers can deduce the mass and rough orbit of the planet doing the wrenching.

Cameron spotted his giant willow world another way. The team is part of a consortium called SuperWasp, which stands for Super Wide Angle Search for Planets, run by Don Pollacco at Queen's University in Belfast. Together, they operate two ground-based telescopes, one at the summit of La Palma, a volcanic shard of an island in the Canaries, the other on a hilltop at Sutherland in the Karoo desert, a few hours drive north east of Capetown.

Every evening, automatic covers roll back from the telescopes to reveal the stars, which are snapped every 30 seconds as they curl over the sky from sunset to sunrise. In a 10-minute period, each telescope will capture the light of 3.5m stars.

In the mornings, the previous night's images from La Palma are piped down the internet to Belfast. Those from Africa are stored on magnetic tapes and shipped back in bulk. (The internet can be less reliable there.)

At Pollacco's lab, a bank of computers then spends 12 hours a day examining every image of every star, looking for telltale dips in the brightness caused by a planet moving across the face of its sun. The most promising candidates are then beamed out to researchers such as Cameron for confirmation.

In 2005, the SuperWasp team found two huge planets this way, the willow world, in the constellation of Andromeda, and a smaller, more dense planet circling a star in the constellation of Delphinus, the dolphin. Last week, the team announced its latest discoveries - three more giant planets more than 700 light years away that pass so close to their suns that their surfaces boil.

The SuperWasp technique, known as the transit method, has its shortcomings, though. Only a tiny percentage of planets orbit their stars in the right plane to cast their shadows across the Earth, and only giant worlds that come close to their stars produce clear enough signals to reveal themselves.

But this is a fast-changing field: improvements and refinements are bringing astronomers ever closer to their ultimate goal of finding a second Earth. Earlier this year, scientists led by Stephane Udry at the European Southern Observatory in Geneva discovered a planet orbiting a faint red dwarf star in the constellation of Libra, 20 light years away. Their measurements suggest the planet, Gliese 581c, is small and rocky - only one and a half times the size of Earth. Its orbit has led some scientists to speculate that liquid water might be stable on its surface, raising the tantalising possibility that it could support life.

If life does exist on Gliese 581c, it has yet to generate enough noise to be noticed by its galactic neighbours, however. Scientists at the California-based Search for Extraterrestrial Intelligence (Seti) project have looked for radio waves emanating from the star system - the Gliese equivalents of 20-year-old TV broadcasts - and found nothing.

Just three months after Udry's announcement, a landmark discovery by Giovanna Tinetti and her team at University College London sent a further ripple of excitement through the field. They used Nasa's Spitzer Space Telescope to peer at infrared radiation coming from a star called HD 189733 that lies 63 light years from Earth. When a newly discovered planet moved close to the star, they noticed a familiar pattern in the infrared signal. The discovery, reported in the journal Nature, marked the first incontrovertible evidence of water in the atmosphere of a planet beyond our solar system. (Nasa's mantra for finding extraterrestrial life, by the way, is: "Follow the water." It is the one ingredient that scientists believe is crucial for life as we know it.)

In the canteen in the physics building at St Andrews, a substantial pile of napkins marked with giant circles and squiggly light rays has been moved to one side and the discussion has shifted from orbital mechanics to more philosophical ground. In the coming years, expectations are running high that planet hunters will stumble across a second Earth, at least at first glance. What then? Astronomers will undoubtedly point everything they have at it in a desperate bid to find signs of life. "Will the answer make us feel comforted that we're not alone, will it make us feel privileged and special, or lonely and distant?" asks Cameron. "It'll be an interesting time for the Creationists," adds his colleague and fellow planet hunter Alan Penny.

If this year looked prolific for the planet hunters, there is good reason to believe that many more discoveries are coming. Last December, the French space agency, CNES, launched Corot, the first space-based telescope dedicated to hunting planets and it has already reported its first discovery, a large Jupiter-like planet 1,500 light years away. But this was during little more than a glimpse at the heavens. "The advantage of being in space is that you don't have to look through Earth's atmosphere, you don't have interruptions due to bad weather and you can look continuously, because there is no day and night problem," says Suzanne Aigrain, an astrophysicist at Exeter University, and the leading British scientist working on the Corot project.

The Corot team believe they will spot planets only a few times larger than Earth, revealing at the very least just how common planets such as ours are in the galaxy. As with the SuperWasp team, the Corot scientists are in the process of confirming the discovery of a series of new planets and expect to announce them later in the year.

Elsewhere, scientists are building new telescopes and redirecting existing ones to join the search. In the Arizona desert, a team led by David Charbonneau, the Harvard-Smithsonian Centre for Astrophysics scientist who discovered the first transitting extra-solar planet in 1999, is preparing MEarth, a cluster of eight new telescopes to look for Earth-like planets around stars called M-dwarfs.

"What we yearn for now are the small, rocky, pipsqueak planets like Earth, and that's where the race is," says Charbonneau. "The first step is to find those that are similar in size to Earth and have a solid surface, somewhere where there might be life."

In 2009, Nasa is set to launch its Kepler mission, a space observatory that will trail the Earth and stare out to space looking for similar planets. The European Space Agency is putting together its own mission, Darwin, that will be the first to look not just for planets, but for signs of life in their atmospheres. Oxygen, for example, is not thought to hang around in an atmosphere unless there is life to keep churning it out. On Mars, the oxygen that once lurked in the atmosphere has become locked up in the rocks, turning elements into oxides.

"We have to admit there may be life that's very different to that on Earth, but we have to begin by looking for planets that are like ours and then for signatures of life in those atmospheres," says Charbonneau. "The universe may be full of life that is very different, but whether we would recognise it from afar is an open question. The big change is that if 10 years ago I had said I was looking for life I would be classified as some kooky astronomer, but things have changed. It's now a legitimate question."